We have prepared populations of antigen-specific B cells, 80% of which bind to TNP (TNP-ABC). Using both thymus-independent (TI) and thymus-dependent (TD) antigens (the latter in the presence of Th cells), we have defined the conditions for inducing cell enlargement, entry into cycle, proliferation and differentiation of the TNP-ABC. Preliminary studies have indicated that TNP-ABC from long-term primed mice will proliferate and differentiate into IgG secreting cells. Based on this observation, we will prepare TNP memory ABC (TNP-MABC) and study their phenotype and activation requirements using TI and TD antigens as well as polyclonal activators (e.g. anti-Ig). The studies will be designed to determine how and why memory cells are so effectively activated by antigens to clonally expand and secrete IgG. The parameters to be studied will include efficiency of receptor-mediated crosslinking and triggering, number of Th cells required, their responsiveness to cytokines and the frequency and burst size of IgG-secreting cells. The results of such studies should shed light on the differences in triggering requirements for virgin vs memory antigen-specific B cells.